1. Field of the Invention
The present invention relates to an electrolytic capacitor. More particularly, the present invention relates to an electrolytic capacitor including a capacitor element having a pair of electrode foils wound with a separator interposed therebetween.
2. Description of the Related Art
As digitization of electronic equipment proceeds, it has become required that capacitors used therefor have a smaller size, a larger capacitance and a lower Equivalent Series Resistance (hereinafter, referred to as “ESR”) in a high-frequency range.
In order to reduce the ESR in a high frequency range, a solid electrolytic capacitor using, an electrically conductive material such as an electrically conductive polymer containing polypyrrole, polythiophene, and derivatives thereof as an electrolyte with higher electric conductivity than that of a conventional driving electrolytic solution has been known (see, for example, Japanese Examined Patent Publication No. 3040113).
Further, in terms of an increase in the capacitance, a wound solid electrolytic capacitor having a configuration in which a capacitor element having an anode foil and a cathode foil wound with a separator interposed therebetween is filled with an electrically conductive solid layer containing the above-described electrically conductive solid polymer has been commercialized.
However, with respect to the conventional wound solid electrolytic capacitor, since an electrically conductive solid polymer having low repairability of a dielectric layer as an electrolyte is employed, there is not only a tendency to decrease the withstand voltage and increase the leak current but also a tendency to cause a short circuit, and the like, along with an increase in the sudden leak current and damage of the dielectric layer during the use.
Therefore, a wound solid electrolytic capacitor, in which a capacitor element contains both of an electrically conductive solid layer containing an electrically conductive solid polymer and an electrolytic solution, has been proposed (see, for example, Japanese Unexamined Patent Publication No. 2006-100774).
The conventional electrically conductive solid layer containing the electrically conductive polymer in the capacitor element is produced in the following manner. First, an anode foil having a dielectric layer thereon and a cathode foil are wound with a separator interposed therebetween to form a capacitor element. Next, this capacitor element is impregnated with a polymerization solution containing a polymerizable monomer such as pyrrole, thiophene, and derivatives thereof, an oxidizing agent such as ammonium persulfate, and sodium persulfate, and a doping agent such as naphthalenesulfonic acid. Then, the above-mentioned polymerizable monomer is oxidatively polymerized in the inside of the capacitor element to form the electrically conductive solid layer.
However, the inventors of the present invention have made studies, and thus have found that since in the method of forming an electrically conductive solid layer in the inside of a capacitor element by polymerization reaction, as described above, the oxidizing agent adheres on the anode foil to perform oxidative polymerization on the dielectric layer, the dielectric layer on the anode foil is easily damaged and the electrically conductive solid layer tends to be formed unevenly on the surfaces of the electrode foils and the separator. As a result, there are problems of inferiority in the solder heat resistance in the mounting process and considerable deterioration of the capacitor properties in a long time use. Further, the above-mentioned method requires a process of removing the unreacted polymerizable monomers and the oxidizing agent by washing after the oxidative polymerization, and thus there is a problem that the process becomes complicated.